A transmission, including a plurality of planetary gear trains, has been recently installed in the construction vehicles such the dump trucks. In the gearbox, engine torque is inputted into an input shaft and a plurality of speed stages are shifted back and forth depending on objects of driving a vehicle.
For example, U.S. Pat. No. 4,205,563 (issued on Jun. 3, 1980) discloses a gearbox in which not only clutches and brakes but also a torque converter and two sets of planetary gear trains are combined. According to the gearbox, a wide range of speed reduction can be realized by setting the inter-stage ratios to be roughly the same. Note the inter-stage ratio is a ratio between speed change ratios of speed-change stages.